Field of the Invention
This invention relates to gasification of carbonaceous material, and more particularly to a two stage entrained-bed gasification process and apparatus therefor for gasifying coal.
Treatment of carbonaceous material such as, for example, coal with heat and pressure in order to drive off the volatile components and provide solid, liquid and gaseous products for fuels and chemicals, has been carried out by several processes for over a century.
This technology was used as early as 1807 when town gas produced from coal lit a public street in London. By the turn of the century, German chemists were making a number of products from coal. A large part of the WW II German war machine was fueled by gasoline made from coal. Low Btu gas from coal was also widely used in the United States before the advent of cheap natural gas and oil. Cheap gas and oil pushed coal gasification technology aside, and it did not undergo any major technological advancements until it reemerged recently because of, among other things, substantial increases in the cost of natural gas.
The gasifier or reactor is the heart of a coal gasification process and there are four main types of gasifiers, all of which rely upon external sources of heat or the burning of part of the coal to provide the heat needed to effect gasification.
One well-known type of gasifier, of which the Lurgi device is typical, is the fixed-bed gasifier. In this type of gasifier, sized coal is supplied to the top of the gasifier and the gasifying medium such as oxygen and steam is injected at the bottom. Such gasifiers utilize the lowest operating temperatures and require long residence times of up to 1 hour. Due to the low temperatures used, large amounts of heavy liquids are produced. Ash is removed from the bottom of the gasifier as dry ash or slag depending on the operating temperature. For slagging operation, the gasifier is run at comparatively higher temperatures thus requiring more oxygen and less steam, but providing a faster reaction rate than for the non-slagging mode of operation.
Inherent advantages of a fixed-bed process are high thermal efficiency and carbon conversion and low contamination of gas with solids. Among the disadvantages are that caking coals cannot be used without pretreatment. The coal must have uniform size and good mechanical strength. Production of heavy hydrocarbons is undesirable if the gas produced is to be used as synthesis gas or to produce high Btu gas.
A second type of gasifier is the fluidized-bed gasifier which operates with crushed or fine coal. The fluidized-bed gasifier as compared to the fixed-bed gasifier allows improved gas-solid mixing, uniform temperature distribution and improved gas-solid contact. Fluidized-bed gasifiers can tolerate variations in coal feed during operation, have high gasification rates per unit cross-sectional area and can operate over a large range of output without significant loss in efficiency. Fluidized-bed gasifiers in general require pretreatment of caking coals and longer residence times when compared with entrained-bed gasifiers discussed below. Temperatures are lower than entrained-bed gasifiers, but higher than fixed-beds. Exit gases generally have high dust loading and the range of operating conditions is limited because of fluidization characteristics of particles and danger of entrainment.
A third type of gasifier is the molten bath (salt or iron) gasifier wherein coal is fed with oxygen and steam into a molten bath. Ash and other impurities float to the top as slag and are removed.
The fourth type of gasifier is the entrained-bed which may be divided into single stage and two stage types.
The single stage type is sometimes referred to as the partial oxidation gasifier. In this type, pulverized coal and the gasifying medium, typically oxygen and steam, are fed cocurrently and the coal is gasified in more or less suspension. The exit gas has little or no tars or methane because at the high temperatures used the homogeneous gas-phase reactions proceed to thermodynamic equilibrium. To run the gasifier at high temperatures, larger amounts of oxygen are required compared to fluidized or fixed-bed types. The exit gases have high temperatures and high loading of ash particles. Overall fuel-gas production rates per unit volume of gasifier space are higher than in fluidized or fixed-bed types because of both high reaction temperatures and large particle surface area.
The two stage entrained-bed gasifier, developed at Bituminous Coal Research, Inc., Pittsburgh, PA in the 1960's has perhaps the greatest potential for development of known gasification processes. The present invention is an improvement of the two stage entrained-bed gasifier.
In the two stage type, pulverized coal is introduced into a second or gasifier stage to produce a process gas and a process char. This process char is separated from the process gas and recycled and reacted with oxygen and steam in a first or combustion stage to produce hot combustion gas. As used herein "combustion gas" includes predominantly carbon dioxide and water vapor with lesser amounts of hydrogen and carbon monoxide. The hot combustion gas from the combustion stage is introduced into the aforementioned second stage and contacts the pulverized coal introduced into the second stage. Here the coal is heated and reacted in contact with the combustion gas and steam to produce synthesis gas, some methane, and process char. This gasification reaction is carried out typically at low gas flow velocities of the order of 2-12 feet per second, pressures of about 60 atmospheres and temperatures of about 1200.degree. K.
The pressure and temperature of the combustion gas produced in the first stage are such that in the second or gasifier stage, the classic heterogeneous carbon/steam and carbon/carbon dioxide reactions take place to produce CO and H.sub.2.
Upon issuing from the second stage, the exiting gases and entrained char are passed into a quenching zone to cool the gas and char to below the reaction temperature. Thereafter, the quenched process stream is separated into its gaseous and char components.
This process and apparatus have the ability to produce a tar-free, low-sulfur content char product in addition to a gaseous product. For a more complete discussion of the two stage entrained-bed gasifier, reference is made to Department of Interior, Office of Coal Research publication, dated 1965 and entitled "Gas Generator-Research and Development Survey and Evaluation" prepared by Bituminous Coal Research, Inc.; "An Evaluation of the BCR Bi-Gas SNG Process", W. P. Hegarty et al, Chemical Engineering Progress, Vol. 69, No. 3, March 1973; U.S. Pat. No. 3,746,522, issued July 17, 1973; U.S. Pat. No. 3,782,913 issued Jan. 1, 1974; U.S. Pat. No. 3,840,354 issued Oct. 8, 1974; and U.S. Pat. No. 3,844,733 issued Oct. 29, 1974, all of which are incorporated herein as if set out at length.
It has become known in recent years, from the data of experiments performed by ourselves and others, that if coal particles are subjected to very high heating rates, of the order of 10.sup.5 .degree. K./sec and higher, a much larger fraction of the coal mass may be devolatilized than the so-called "volatile matter" content of the coal as defined by ASTM Proximate Analysis. In view of the rapidly-changing and somewhat inhomogeneous conditions in such experiments, it is customary to express properties such as velocity, temperature and heating rates in terms of suitable spatial or temporal averages. The cited very high heating rate of 10.sup.5 .degree. K./sec or higher is such an average over the brief period of devolatilization.
The value of heating rates on the order of 10.sup.5 .degree.K./sec and higher has been documented in reports of laboratory experiments, viz:
(1) Kimber, G. M. and Gray, M. D., "Combustion and Flame", 11, 360, (1967). PA1 (2) Ubhayakar, S. K., Stickler, D. B, von Rosenberg, C. W., Jr., and Gannon, R. E., "Rapid Devolatilization of Pulverized Coal in Hot Combustion Gases", 16th Symposium (International) on Combustion, 427 (1976).
When done under well-mixed conditions with high temperatures (T.gtoreq.1400.degree. K.), such large heating rates were shown to lead to larger yields of volatiles than conditions with slower heating rates. However, a potential benefit resulting from such heating rates, recognized by us and enjoyed by our invention is a reduction, or even elimination, of the requirement for heterogeneous gasification reactions, which are slow and inefficient. Consequently, use of high heating rates can lead to smaller amounts of oxygen consumption for the total process.
It must be pointed out that the above-noted data were obtained under laboratory conditions, using means or methods not practical for commercial gasification. Existing two-stage gasifiers have residence times which are at least two orders of magnitude larger than those required for operating with the higher degree of devolatilization. We perceive that the reason for this is as follows: a coal particle must be very small if it is to be heated rapidly, even in a very high temperature gas. But heretofore such small particles were permitted to mix slowly with respect to the entraining hot gas, so that heat is brought to them relatively slowly. Such heating is "mixing limited". Heating under "mixing limited" conditions occurs when the characteristic mixing time is greater than the characteristic time for diffusive heat flow to the coal particles and for thermal diffusion within the particles. Similarly, any volatiles arising from such a particle were also permitted to tend to remain near the particle, and to degrade to soot rather than reacting with the surrounding gas to form stable hydrocarbons. Such stabilization is also mixing limited. And the final attainment of equilibrium of the heterogeneous reaction between coal and soot particles and the surrounding gas is also mixing limited. As a result, the present state of the art in two-stage entrained-bed gasifiers is such that the heating rate of the carbonaceous matter particles and the residence times for reactants in the gasification stage are mixing limited. For one example, the gasifier described in the aforementioned Donath U.S. Pat. No. 3,782,913 depends on high pressure, residence times of 5 to 15 seconds and equilibrium chemistry to yield product gas containing essentially the equilibrium amount of methane.
In one set of our experiments, for example, coal was subjected to steam at 1370.degree. K. and 10 atm pressure for reaction times of 50 milliseconds and generated methane in excess of that expected based on equilibrium calculations. Further data obtained by us have shown that under conditions of rapid heating to temperatures of 1370.degree. K. and higher, of finely pulverized coal well-dispersed in a background of steam, followed by rapid cooling, one can obtain methane concentration in the product gas which is substantially larger than would be predicted by equilibrium considerations for the experimental reactor conditions. The detailed reaction chain leading to this is not known, but it is well-known that to attain an equilibrium composition in any chemical reactor requires adequate time. The experimental conditions provided initial temperatures and reaction times which were sufficient for pyrolyzing large amounts of mass from the coal, but at later stages, the temperature-time history was inadequate for attaining equilibrium among the gas phase constituents.
We believe this enhanced volatilization present in accordance with our invention is attained through a nonequilibrium rapid direct pyrolysis pathway, rather than through the usual prior art heterogeneous reaction process. Our invention is thought to give hydrocarbon radicals which react homogeneously with background gas to yield a nonequilibrium product distribution which can be retained by sufficiently prompt cooling. Whatever the reason may be, it is clear that extremely rapid heating of coal particles yields copious amounts of volatiles. This is preferably done in the presence of gases which react with and stabilize the volatiles to prevent formation of soot, and with sufficiently prompt cooling to prevent shift of the composition to equilibrium values.